1. Field of the Invention
The present invention relates generally to annulus pressure responsive downhole tools, and particularly to a combination safety-circulating valve operated by a differential area piston referenced to well annulus hydrostatic pressure.
2. Description of the Prior Art
When an oil well is drilled, it is often desired to test the production capabilities of the subsurface formations intersected by the well by lowering a testing string into the borehole to the formation depth. The formation fluid is then allowed to flow into the test string in a controlled testing program.
It is now well known in the art to operate one or more of the tools in the test string in response to increases in well annulus pressure in a well annulus between the test string and the well borehole. This is often far superior to using pipe manipulation through rotation or reciprocation to operate the testing tools, particularly in deviated boreholes such as are drilled from offshore platforms.
One testing tool which is commonly included in the test string is a combination safety and circulating valve.
Such a combination safety and circulating valve which has been utilized by the assignee of the present invention is disclosed in U.S. Pat. Nos. 4,270,610 to Barrington, 4,311,197 to Hushbeck, and 4,445,571 to Hushbeck.
The device shown in the three patents just referenced is generally referred to as a combination sampler valve and circulation valve. The term sampler is utilized because the tool disclosed in these three patents utilizes two spaced ball valves which can trap a sample of the flowing fluid therebetween. The ball valves themselves, however, can also be referred to as safety valves since they operate to shut off the flow of well fluid through the test string.
Although the apparatus disclosed in the present application includes only a single ball type safety valve, it will be understood that it could be modified to add a second ball and thus be a sampler valve, and the prior art sampler valves disclosed in the three patents referenced above could have the lower ball thereof eliminated so that they would then provide only a safety valve and circulating valve.
The prior art combination sampler and circulation valve disclosed in the three patents cited above is referred to as an atmospheric referenced tool. That is, the differential area piston which operates that tool has a low pressure side exposed to substantially atmospheric pressure. Referring for example to U.S. Pat. No. 4,270,610 to Barrington, and particularly to FIG. 2B thereof, a sealed low pressure chamber 80 is thereshown which contained air at atmospheric pressure when the tool was first assembled before running into the well. Although that pressure may change due to heating or cooling after the tool is placed in a well, this is still generally referred to as an atmospheric referenced tool.
The tool shown in FIGS. 2A-2F of U.S. Pat. No. 4,270,610 is utilized in a test string as illustrated in FIG. 1 of that patent, and generally has an annulus pressure responsive tester valve located in the same string therebelow.
Generally, the test string is lowered into a well, and then after a packer of the test string is set, well annulus pressure may be repeatedly increased and then dropped back to hydrostatic pressure to operate the well tester valve located below the combination sampler-circulating valve. The sampler-circulating valve is designed to operate at a higher differential pressure between the well annulus and the interior of the test string than is the tester valve located therebelow.
After the testing program is completed, well annulus pressure is then increased to the higher level necessary to operate the sampler-circulating valve, and the two ball valves of the sampler section will then be closed to trap a flowing sample of well fluid and to close the bore of the test string against further flow of well fluid therethrough while at substantially the same time a circulating valve above the sample chamber is opened to communicate the interior of the test string with the well annulus.
The power mandrel of the combination sampler-circulating valve of U.S. Pat. No. 4,270,610 is retained in place against premature operation by a shear set 100 seen in FIG. 2B thereof which includes a large plurality of shear pins 112. The shear set is designed to shear when the difference between well annulus pressure and pressure interior of the test string reaches a predetermined level at which it is desired to operate the sampler-circulating valve.
The shear pins of the shear set must be designed to hold against the hydrostatic well annulus pressure plus the increase in well annulus pressure which is utilized to operate the tool. This increase in well annulus pressure is generally in the range of 1500 to 2500 psi.
As will be well understood by those skilled in the art, the hydrostatic well annulus pressure which is present due merely to the weight of the drilling mud contained in the well bore may itself be on the order of 10,000 psi. Thus, the shear pins of the shear set 100 of the U.S. Pat. No. 4,270,610 must be designed to hold the power mandrel in place against the difference between hydrostatic well annulus pressure of perhaps 10,000 psi and the substantially zero pressure in chamber 80 for long periods of time during the testing program, and must then reliably fail at an increased pressure differential of 1500 to 2500 psi.
Thus, the shear pins of the shear set must support 80% to 90% of the designed shearing load for long periods of time while being subjected to high temperatures, and often to corrosive environments in the well. It is common for brass shear pins to stress crack due to corrosion caused by ammonia present in the well.
This leads to substantial problems due to inconsistent operating pressures of tools such as those shown in U.S. Pat. No. 4,270,610.
The problem is due in part to the variation in shear strength of the shear pins themselves which are generally constructed of brass. Quality control requirements governing the production of the pins is very stringent, but if a large number of pins is required to be used on a job, such as illustrated in FIG. 2B of the U.S. Pat. No. 4,270,610, the actual shear pressure may be significantly different than calcuated.
Additionally, the number of pins required for a specific job is determined by the depth at which the tool is run and the mud weight, that is the weight of the drilling fluid contained in the well. Many times the mud weight value may be incorrectly stated and therefore calculations can be off considerably.
The design of the U.S. Pat. No. 4,270,610 therefore depends heavily upon the shear pins for proper operation, where in fact many variables exist which can substantially alter the operating pressure of the tool at which the shear pins will shear.
The reason so many shear pins are required in tools such as those shown in U.S. Pat. No. 4,270,610 is that the tools are referenced to substantially atmospheric conditions and thus the pins must resist the hydrostatic well annulus pressure plus approximately 2500 psi.
Additionally, although the design of U.S. Pat. No. 4,270,610 using a large number of pins most often has a problem with too low of an operating pressure due to deterioration of the pins as described, it can also have a problem with too high of an operating pressure due to a build-up of tolerances in construction of the pins.